As it is easily reproducible could you repeat setting a new movie that gets an open subtitles srt file - only this timesavers sysinternals process monitor running to capture all events and and try to stream and when you get the error, stop procmon capture and get the logs as before
Save the procmon capture as PML file and zip and upload to Dropbox etc and send me link by private message and also get the server logs
And let me know the time of the first event in the PML as timeline differences would result in me not seeing the real times of events when i open the PML file - note that procmon produces large files so stop the capture as soon as the error occurs and start the capture before opensubtitles gets the SRT.
Procmon link Process Monitor - Sysinternals | Microsoft Learn
In theory disabling the agent and removing it from the advanced settings for the library would do this - but may need to go through the Plex Dance sequence as well - effectively recreating the items in the library
